The liquid crystal display devices have a distinguishing characteristic allowing operation at a low electric power comsumption and due to this advantage, liquid crystal devices are used in various fields including OA machines.
Use of organic dispersing electroluminescent devices (hereinunder referred to as "EL devices"), which are thin and light-weight, is becoming wide spread for inexpensive lights (auxiliary light sources) of plane-luminescence used for liquid crystal devices.
Since EL devices include fluorecent materials such as Zns:Mn, Zns:Cu etc., the EL devices are packaged in a transparent film which has a high resistance to moisture because the luminance is greatly lowered by moisture.
Two laminated moistureproof films each having a sealant layer on one side thereof which can be heat-sealed, are generally used as a packaging film for EL devices. An EL device which is inserted between the sealant layers of the two laminated moistureproof films, is passed through a vertical pair of heated rollers at an appropriate nip pressure so as to heat-seal the sealant layers surrounding the EL device.
A laminated film having a fluorinated resin film, especially, a film containing polychlorotrifluoroethylene (hereinunder referred to as "PCTFE") as the main ingredient is often used as a packaging film due to its excellent resistance to moisture and transparency.
Such a laminated film is generally produced by laminating polyolefin having 20 to 100 .mu.m in thickness as a sealant for heat-sealing on a PCTFE film having 70 to 300 .mu.m in thickness, and it is said to have the highest resistance to moisture of all the existent transparent films because it hardly allowing steam to permeate.
A transparent laminated plastic film produced by forming a transparent metal oxide especially, silicon or aluminum oxide thin film on the surface of a transparent plastic base film, has recently been commercialized as a packaging material having a gas barrier-property.
However, the laminated film having a film containing PCTFE as the main ingredient is so expensive that the manufacturing cost of a back light becomes high. In addition, since the moisture resistance of such a laminated film is lowered when the ambient temperature exceeds 50.degree. C., the life of the EL device packaged therein is extremely shortened when it is used at a high temperature.
On the other hand, the moisture resistance of the transparent laminated plastic film having a transparent metal oxide thin film on the surfaces thereof is insufficient, so that it has not been used as a packaging film for an EL device yet. In addition, the present inventors have found that the packaging films are treated by heat-rollers of 110.degree. to 150.degree. C. in the EL device sealing process, however according to some of the base films used for these packaging films, the moisture resistance thereof deteriorates by the heat-treatment.
Development of a transparent plastic film which is suitable to a packaging film and which is cheaper and has higher moisture resistance than a PCTFE film, has therefore been demanded.